Optical study of the magnetic phase diagram of metamagnetic ferrous bromide

Magnetic ordering in the metamagnet ferrous bromide has been investigated by a variety of magneto-optical techniques. The optical spectrum in the region of the spin-forbidden crystal field transitions (12000-20000 cm -1 ) closely resembles that of FeCl 2 . The principal features have been assigned and a detailed field and temperature-dependent study (0-5 T, 4.2-15K) has been made of the band at 5026 AA, whose assignment as a magnon-exciton combination band is confirmed. Like a similar band in FeCl 2 , its intensity varies as T 2 exp(-A'/k B T) in the range 5-10K, giving the zone-centre gap in the magnon spectrum A' as 17.0+or-1.8 cm -1 in agreement with neutron scattering experiments of Yelon and Vettier (1975). The intensity of the 5026 AA band also varies with an applied magnetic field (H//c) and at 10K reaches a maximum at 3.0 T, close to the metamagnetic transition field, reflecting a maximum in the sublattice disorder. The boundaries of this region have been mapped in the (H, T) plane. Using a transmission technique, the Faraday rotation of FeBr 2 has been measured in units of 90 degrees to angles exceeding 3000 degrees .